Liquid Dye Saturable Absorber

The distinction between an organic dye suitable for simultaneous mode-locking and Q-switching, as opposed to only Q-switching the laser, is the recovery time of the absorber. If the relaxation time of the excited-state population of the dye is on the order of the cavity round trip, that is, a few nanoseconds, passive Qswitching will occur, as described in Chapter 8. With a dye having a recovery time comparable to the duration of mode-locked pulses, that is, a few picoseconds, simultaneous mode-locking and Q-switching can be achieved. A typical example of a mode-locked laser utilizing a saturable absorber is illustrated in Fig. 1.
One major requirement in the resonator design of a mode-locked system is the complete elimination of reflections that can occur from components located between the two cavity mirrors. This is accomplished by employing laser rods with Brewster’s angle at the ends, placing the dye cell at Brewster’s angle in the resonator, and by using cavity mirrors which are wedged. Reflection from an optical surface that is parallel to the cavity mirrors will create a secondary resonator.
The mode-locked pulse will be split into several pulses that will circulate inside the resonators with different round-trip times. The result is a very erratic output usually consisting of several superpositioned pulse trains or containing subsidiary pulses in the train. With all optical surfaces inside the resonator, either at Brewster angle or antireflection-coated and tilted away from the resonator axis, coupled resonator structures can be avoided and the occurrence of satellite pulses is mini-

FIGURE 1. Mode-locking with a liquid dye saturable absorber.

mized. Similar attention must be paid to avoid back-reflection into the cavity from external components.
Despite the relatively simple construction of a passively mode-locked laser oscillator, the output will be very unpredictable unless dye concentration, optical pumping intensity, and resonator alignment are carefully adjusted. Furthermore, mixing and handling the dye solution and maintaining proper dye concentration proved cumbersome. As a result of the inherent shortcomings of pulsed passive mode-locking, this technology has been superseded by cw mode-locking.